In the past, clinicians have sometimes complained about the magnitude of the thumb force (sometimes called the “retraction force,” “activation force” or “total force”) that is required to initiate needle retraction in syringes having retractable needles. When using conventional syringes having needles that are retracted by the expansion of a compressed spring, an additional thumb force is typically applied axially at the back of the plunger handle following injection to initiate needle retraction. The magnitude of the thumb force that is required depends upon the structure and operation of each particular syringe. One part of the required thumb force is attributable to the force needed to break, rupture, remove or displace the cover over the opening into a retraction cavity inside the plunger handle. Another part of the required thumb force is attributable to the force needed to release whatever structure is holding the spring in compression. If the required thumb force is too great, especially for clinicians having small hands, it can be difficult to retract the needle after an injection. This can cause the user to move or twist the syringe while a needle is still inside the patient in order to gain more leverage. Also, unless the structural elements of the device are configured so that the thumb force can be applied smoothly so as to open the retraction cavity and release the compressed spring without jerks or abrupt movement prior to retraction.
Some prior art syringes have utilized a plug or substantially rigid barrier to block the entrance to a retraction cavity inside the plunger handle prior to retraction, but the force required to dislodge or displace the plug, or to fracture the substantially rigid barrier, also increased the required thumb force. Other prior art syringes have used an axially fixed plunger seal having a rubbery portion covering the opening to the retraction cavity in combination with a two-part plunger handle in which an inner member had to be moved past a detent to advance a cutter that severed the covering portion, thereby also increasing the required thumb force. In such cases, the force required to dislodge a plug, fracture a barrier or move a plunger section past a detent not only increased the thumb force required to initiate retraction but also caused the syringe to jerk or move abruptly prior to needle retraction.
U.S. Pat. No. 5,053,010 (McGary) discloses a syringe having a retractable needle and a plunger with an axially slidable plunger seal having a transverse web portion that seals the opening into the plunger retraction cavity prior to retraction. That syringe has a cutting tool disposed on the forwardly extending tip of the plunger that cuts through both the transverse web across the opening to the retraction cavity and another circumferentially extending tab that secures the needle retainer to the barrel wall. Because of the cutter positioned behind the transverse web, there is a risk of premature rupture of the web during injection, which is not desirable because the remaining portion of medicine in the syringe could flow back into the retraction cavity rather than into the patient.
U.S. Pat. No. 5,064,419 (Gaarde) discloses a syringe having a retractable needle and a piston with a piston packing that is confined to prevent axial movement relative to the piston. FIG. 3 discloses a thin membrane attached to the piston packing. The thin membrane seals off a cavity inside the piston until the flat head of the needle holder punctures or destroys the membrane for retraction. The needle holder is held in place by the closed bottom end of a barrel section that is disposed inside the spring retainer.
U.S. Pat. No. 5,180,369 (Dysarz) discloses a syringe having a retractable needle and a gasket disposed between the plunger and the barrel that is axially slidable on the plunger. That syringe has a shatter plate that seals the opening into the retraction cavity inside the plunger prior to retraction. The needle holder is held in place prior to retraction by a shatter plate and shatter ring, each of which requires an additional gasket to provide a fluid seal.
U.S. Pat. No. 5,180,370 (Gillespie) discloses a syringe having a retractable needle and an annular seal ring that provides a fluid seal between the plunger and barrel. The annular seal ring does not appear to be confined to prevent axial movement relative to the plunger but does not appear to slide axially relative to the plunger during use. The plunger has a hollow chamber that is sealed from the interior of the barrel by a resilient rupturable cover.
U.S. Pat. No. 5,201,710 (Caselli) discloses a syringe having a retractable needle held by a needle holder having a head with a sharp part shaped like a crown with a sharp edge that first bends and then breaks a diaphragm connected to a plunger seal element that is confined to prevent axial sliding relative to the plunger cylinder.
U.S. Pat. No. 5,578,011 (Shaw) discloses a syringe having a retractable needle and an elastomeric friction ring to hold the needle holder inside the barrel and to provide a fluid seal between the barrel wall and the needle holder. That syringe has a plunger seal that is confined from sliding axially along the plunger, and an elastomeric plug that provides a releasable seal inside the front opening to the retraction cavity inside the plunger.
U.S. Pat. No. 6,015,438 (Shaw) discloses a syringe having a retractable needle, an annular retainer member to hold the needle holder inside the barrel and an a plunger seal disposed between the plunger and the barrel that is axially slidable along the plunger for retraction. That syringe has a separate, removable stopper that seals the opening into the retraction cavity inside the plunger prior to retraction.
U.S. Pat. No. 6,994,690 (Kiehne) and U.S. Pat. No. 7,544,182 (Kiehne) disclose syringes having a retractable needle and a tapered extension on the inner part of the head of the needle holder that projects rearwardly to burst or pierce a frangible portion of a fixed plunger seal as the plunger is advanced against the needle holder. However, in those syringes the plunger seal is axially confined and cannot slide rearwardly in relation to the front of the plunger to stretch the frangible portion tautly from the barrel side as the plunger contacts the needle holder.
A medical device having a retractable needle is therefore needed that can be operated smoothly to initiate needle retraction and that reduces the thumb force required to initiate retraction